1. Field of the Invention
The invention relates to a superconductor device having a magnet which contains at least one superconductive winding and a refrigeration unit which has at least one cold head.
2. Description of the Related Art
Corresponding superconductor devices are known, for example, from “Proc. 16th Int. Cryog. Engng. Conf. [ICEC 16]”, Kitakyushu, J P, 20. May 24, 1996, Verlag Elsevier Science, 1997, pages 1109 to 1132.
In addition to metallic superconductor materials such as NbTi or Nb3Sn, which have been known for a very long time and have very low critical temperatures Tc, and which are therefore also referred to as low-Tc superconductor materials or LTC materials, metal-oxide superconductor materials with critical temperatures Tc above 77 K have been known since 1987. The latter materials are also referred to as high-Tc superconductor materials or HTC materials.
Attempts have also been made to produce superconductive metal magnet windings with conductors using such HTC materials. Because their current carrying capacity in magnetic fields has until now been relatively poor, in particular with inductions in the Tesla range, the conductors of such windings are often nevertheless kept at a temperature below 77 K, for example between 10 and 50 K, despite the intrinsically high critical temperatures Tc of the materials used, in order in this way to make it possible to carry significant currents with relatively strong field strengths, for example of several Tesla.
Refrigeration units in the form of so-called cryogenic coolers with a closed helium compressed gas circuit are preferably used to cool windings with HTC conductors in the stated temperature range. Cryogenic coolers such as these are, in particular, of the Gifford-McMahon or Stirling type, or are in the form of so-called pulse tube coolers. Refrigeration units of this type furthermore have the advantage that the refrigeration power is effectively available at the push of a button, so that there is no need for the user to handle cryogenic liquids. When using refrigeration units such as these, a superconductive magnet coil winding, for example, is cooled indirectly only by thermal conduction to a cold head of a refrigerator, that is to say without any refrigerant (see also the cited text reference ICEC 16).
At the moment, superconductive magnet systems, in particular MRI (magnetic resonance imaging) installations, are generally cooled by bath cooling, in the case of helium-cooled magnets (see U.S. Pat. No. 6,246,308 B1). A comparatively large amount of liquid helium, for example several hundred liters, has to be stored for this purpose. This amount of liquid helium leads to an undesirable buildup of pressure in a cryostat that is required when the magnet is quenched, that is to say during the transition from the parts of its winding initially being superconductive to the normally conductive state.
For LTC magnets, refrigerator cooling systems have already been produced using highly thermally conductive connections, for example in the form of copper tubes, which may also possibly be flexible, between a cold head of an appropriate refrigeration unit, and the superconductive winding of the magnet (see the cited literature reference from ICEC 16, in particular pages 1113 to 1116). Depending on the distance between the cold head and the object to be cooled, the large cross sections which are required for good thermal coupling then, however, lead to a considerable enlargement of the cold mass. Particularly in the case of magnet systems with a large physical extent, as are normally used for MRI applications, this is disadvantageous because of the extended cooling-down times.
Instead of thermal coupling such as this of the at least one winding to the at least one cold head via thermally conductive solid bodies, it is also possible to provide a line system in which a helium gas flow circulates (see, for example, U.S. Pat. No. 5,485,730).